Dickinsonia belonged to a group of now-extinct life forms called the Ediacaran biota.

While the matter wasn't entirely settled, paleobiologist Doug Erwin said: "I think the consensus among the majority of people who have worked on these fossils for the last decade or so has been they're somehow related to animals".

The first complex organisms emerged during the Ediacaran period (635-541 million years ago). The fossil was found in a remote area near the White Sea in Russian Federation and it was so well preserved its tissue still contained a type of fat that is a hallmark of animal life.

"I took a helicopter to reach this very remote part of the world - home to bears and mosquitoes - where I could find Dickinsonia fossils with organic matter still intact", said Ilya Bobrovskiy, the paper's lead author. These organisms predated the "Cambrian explosion" of animal life over the next nine million years. It lived about 558 million years ago during the Ediacaran period, about 20 million years before the "Cambrian explosion", a period lasting tens of millions of years that has often been considered to mark the emergence of complex animals.

The discovery of the fat confirms that the creature, dubbed "Dickinsonia", is the Earth's earliest known animal, according to the paper.

The problem that was holding researchers back for so long was that Dickinsonia and other Ediacarans have boneless bodies that weren't easily preserved.

A odd fossil that looks a bit like a giant leaf, or a fingerprint the size of a coffee table, has intrigued scientists for decades. Even as the Dickinsonia and fellow Ediacarans yielded to these new species, their legacies remained, permanently preserved in Earth's fossil record.

"I had to hang over the edge of a cliff on ropes and dig out huge blocks of sandstone, throw them down, wash the sandstone and repeat this process until I found the fossils I was after", he added.

"These fossils were located in the middle of cliffs of the White Sea that are 200 to 330 feet (60-100 m) high".

"Paleontologists normally study the structure of fossils, but we extracted and analyzed molecules from inside the Dickinsonia fossil found in ancient rocks to make the breakthrough discovery", Bobrovskiy said.

Bobrovskiy then developed a special technique to test the Dickinsonia remnants for cholesterol. And, since animals are the only organisms capable of producing cholesterol, they argue that the molecules offer definitive evidence of Dickinsonia's status.